1. Field of the Invention
The invention relates to a fuel cell system, and more particularly to a fuel cell system used along with a storage device.
2. Description of the Related Art
For example, a vehicle installed with a fuel cell is provided with a storage device in addition to the fuel cell, and required electric power is supplied from the storage device when the fuel cell is started, or when electric power required by the vehicle cannot be provided solely by the output of the fuel cell.
In the fuel cell, fuel gas, such as hydrogen, is supplied to an anode side, and oxygen-containing oxidizing gas, such as air, is supplied to a cathode side, so that required electric power is generated by reaction between the fuel gas and the oxidizing gas through an electrolyte membrane. At this time, water is formed as a reaction product on the cathode side. Since the water thus formed permeates through the electrolyte membrane and reaches the anode side, the humidity on the anode side of the electrolyte membrane is increased to an appropriate level, and transport of hydrogen protons through the membrane takes place under the humidity, so that electric power is generated by reaction between the protons and oxygen. Accordingly, it is necessary to control the humidity or moisture content of the electrolyte membrane to an appropriate level, so as to effect power generation with high efficiency. Thus, the output of the fuel cell is affected by a water distribution condition sensed at the electrolyte membrane, and a purging operation or hydrating operation is conducted so as to keep the water distribution condition optimized.
While the storage device repeats charge and discharge, its output characteristics deteriorate due to excessive charge or excessive discharge. Also, the output of the storage device varies with the temperature. Thus, since the output of the storage device has a certain relationship with the state of charge thereof, the state of charge (SOC) of the storage device and the storage device temperature are both managed or controlled.
For example, Japanese Patent Application Publication No. 2008-282767 (JP-A-2008-282767) discloses a fuel cell system in which a purging operation is performed based on measured impedance value and detected SOC value. Also, when the fuel cell is in a normal operating mode, the system performs a purging operation for a set period of time, without taking account of the SOC. When the fuel cell is in a low-temperature operating mode, a purging operation using low-humidity gas is performed for improvement of the startability or starting performance.
Also, Japanese Patent Application Publication No. 2007-324071 (JP-A-2007-324071) discloses a fuel cell system that determines purge conditions based on the amount of water remaining in a fuel cell stack, the temperature of a secondary battery that supplies electric power to an air compressor, and the state of charge of the secondary battery, and controls the amount of purge air supplied from the air compressor.
Also, Japanese Patent Application Publication No. 2004-281219 (JP-A-2004-281219) discloses a fuel cell system that operates normally when the maximum electric power required by a load and accessories cam be supplied from a fuel cell and a secondary battery, and performs a warm-up operation when the maximum power cannot be supplied. Here, the warm-up operation is effected by repeating power generation and halts of the fuel cell, and repeating charge and discharge of the secondary battery.
Thus, in the fuel cell systems of the related art, the output of the fuel cell and the output of the storage device are respectively secured, and a target range of a water distribution condition during operation of the fuel cell and a target range of SOC during operation of the storage device are determined. Also, the output of the storage device is utilized when there is a shortage of the output of the fuel cell during operation, and the storage device is charged with power from the fuel cell when there is a shortage of the output of the storage device. Thus, the required power is allocated to the fuel cell and the storage device.
In the meantime, there is no description in the above-identified publications about effective power allocation between the fuel cell and the storage device when the water distribution condition of the fuel cell or the state of charge of the storage device, which falls outside the target range, is brought to within the target range.